patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)

runtime/doc/if_lua.txt

@@ -32,7 +32,7 @@ Examples:
 	:lua local curbuf = vim.buffer() curbuf[7] = "line #7"
 <
 
-:[range]lua << [endmarker]
+:[range]lua << [trim] [{endmarker}]
 {script}
 {endmarker}
 			Execute Lua script {script}.
@@ -40,10 +40,9 @@ Examples:
 			feature wasn't compiled in.  To avoid errors, see
 			|script-here|.
 
-The {endmarker} must NOT be preceded by any white space.
-
 If [endmarker] is omitted from after the "<<", a dot '.' must be used after
-{script}, like for the |:append| and |:insert| commands.
+{script}, like for the |:append| and |:insert| commands.  Refer to
+|:let-heredoc| for more information.
 
 This form of the |:lua| command is mainly useful for including Lua code
 in Vim scripts.